Washington ? The kind of cancer that Sen. Edward Kennedy is fighting is a common, usually incurable form of brain tumor that is very difficult to treat, experts said Tuesday.

About 10,000 cases are diagnosed each year in the United States, and only about half of those patients survive one year, experts said. After two years, perhaps 25 percent are still alive.

“In general, it’s a very grim kind of prognosis,” said Robert Laureno, chief of neurology at Washington Hospital Center. “It’s a bad kind of tumor.”

A key question is exactly which kind of malignant glioma Kennedy has – anaplastic astrocytoma or glioblastoma multiforme, said Lynne Taylor of the Virginia Mason Medical Center in Seattle. Those with anaplastic astrocytoma have a somewhat better prognosis; they survive about three years on average, she said. Doctors will be able to determine what type of tumor Kennedy has by further analysis of a biopsy sample taken at the hospital.

In neither case are the tumors curable, Taylor said: “They always come back.”

Most malignant gliomas diagnosed in older people tend to be more aggressive and less treatable, medical experts said. Kennedy, a Democrat from Massachusetts, is 76.

“Unfortunately, the vast majority – about 70 percent of those cancers – in adults over 50 tend to be the much more malignant type that typically lead to death within 12 to 18 months,” said Harald Sontheimer, a neurobiologist at the University of Alabama at Birmingham.

The left parietal lobe, where Kennedy’s tumor is located, is responsible for a host of crucial bodily functions, including some aspects of speech, as well as sensation and movement on the right side of the body.

“If the tumor is located in very essential parts of the brain, such as those that control speech and motor functions, then trying to perform surgery could leave the patient devastated,” said Vivek Deshmukh, director of cerebrovascular neurosurgery at George Washington University.

Regardless of whether patients undergo surgery, they typically receive radiation and chemotherapy to try to shrink the tumor and control its growth.